Bacteriophage Mu possesses an unusual recombination system which recognizes specific attachment sites near the ends of the mature phage DNA and recombines them with apparently random sequences in the host DHA. The long range objective of this project is to understand, at the molecular level, the mechanism by which this recombination occurs. This objective will be pursued using the following approaches: 1) Studies of lambda phages containing the ends of Mu will be continued by defining the sites and functions of Mu which are needed to cause inhibition and lambda::mini-Mu growth under conditions permissive for Mu gene expression through isolation and characterization of deletion and point mutant lambda::mini-Mu phages. New approaches will be tried to isolate the class of lambda::mini-Mu phages containing the Mu attachment sites close together and to define why these phages were unstable under previous isolation conditions. 2) The DNA sequences important for Mu integration will be defined by isolating and sequencing cis-dominant integration-defective mutants of the lambda::mini-Mu phage. 3) The relative frequency of cointegrate versus simple insertion of mini-Mu will be studied to define the role of vector DNA replication, the effects of Mu lytic versus lysogenic gene expression, and the roles of specific Mu and host genes in the insertion process. These studies will significantly advance our knowledge of the sites and functions involved in Mu integration and the process by which integation occur. This information is important due to its applicability as a model for understanding non-homologous recombination processes which are involved in the spread of antibiotic resistance in bacteria, generation of spontaneous mutations and DNA rearrangements in procarvotes and eucaryotes, possible application to directed gene expression and DNA rearrangement occuring during development in eucaryotes, and the integration of oncogenic viruses. The fundamental role of such processes in normal and abnormal growth is now only being recognized and may have great impact in the future.